1. Field of the Invention
The present invention relates generally to pressure sensors, more particularly to those used for accurate readings in the 0-1 inch of water range. Still more particularly, the invention relates to a relatively inexpensive and easy to fabricate pressure sensor using an elastomer laminate electrode, a rigid dielectric supported electrode and a pair of covers, which in the most preferred embodiment provide prestraining of the elastomer electrode to decrease temperature sensitivity, thereby enhancing system accuracy.
2. Description of the Prior Art
There exists in control or monitoring systems a need to detect pressure conditions in a wide variety of applications. In several systems previously described, such pressure conditions are detected by transducing to an electrical output using a mechanical movement triggered by the pressure condition. The movement effects a change in an electrical output as a result of a change in, for example, resistance, inductance, capacitance or other electrical characteristic.
Early devices employing capacitance technology generally use one of two system types. In the first, a fixed or stationary electrode formed a base for the sensing unit, and a dielectric material, which could be a fluid, was supported thereon. A conductive outer plate was movably mounted on the dielectric and was exposed to the pressure condition to cause a flex of the dielectric, with a corresponding change in the capacitance characteristic. In the second, a conductive diaphragm was movably mounted between a pair of stationary electrode plates for movement parallel to the plates and coupled with a pressure source for corresponding positioning between the two fixed plates to thereby vary the capacitance. These systems were difficult to manufacture and were easily contaminated by environmentally born dust and the like, resulting in extreme cases in shorting of the system. Any leakage of a fluid dielectric in such systems could also dramatically change the system characteristics.
An earlier system developed by the assignee of this invention is described in Thoma, et al. U.S. Pat. No. 3,814,998 issued June 4, 1974 for "Pressure Sensitive Capacitance Sensing Element". The diaphragm element includes thin outer conductive layers on either side of a core of resilient silicone or other dielectric. The conductive layers are made of the same material as the core, with conductive particles embedded therein. One layer is spaced peripherally from the edge of the core layer (to prevent electrical contact between the conducting layers) and the unit is rigidly mounted with metal contacts connected to the conducting layers (e.g. using a silver conducting paint). The element will flex to the side of lower pressure causing a change in the effective conducting area of the opposed conductive layers and a simultaneous decrease in the thickness of the dielectric inner core. As a result, the capacitance of the unit varies as a function of deflection.
In U.S. Pat. No. 3,943,915 issued Mar. 16, 1976 to Severson for "Intracranial Pressure Sensing Device", two thin metal plates serve as the plates of a capacitor, separated by an air dielectric. The plates are spaced from each other and joined together near their periphery with a polyimide ring and epoxy adhesive. The pressure differential across one of the plates causes the air space between the plates to change and results in a change in capacitance.
A "Multiple Transducer" is described in Arkans U.S. Pat. No. 4,152,748, issued May 1, 1979. In this device, the dielectric layer is made of non-conducting sponge rubber. A first plate of a capacitor is a flexible, conducting layer coextensive with the dielectric, while the second plate of the capacitor is smaller in size than the first plate and has a shape that will best define a localized stress point. There are usually multiple second plates of different size and shape in this device.
Another pressure sensing system is described in Lee U.S. Pat. No. 4,168,517 issued Sept. 18, 1979 for "Capacitive Pressure Transducer", in which the pressure transducer has a capacitor configuration. The plates of the capacitor are metal and the dielectric layer is air. This patent focuses on the use of a deep annular convolution in one of the metal capacitor plates to prevent slipping in the clamping area and the use of an elastic mounting ring to also aid in preventing slipping. The metal plate that defines the area of the capacitor is integrally bonded to a rigid ceramic which is not part of the dielectric layer of the capacitor.
Yet another sensor is disclosed in U.S. Pat. No. 4,177,680 issued Dec. 11, 1979 to Coleman for "Dual Pressure Sensor". The two halves of the sensor are identical in structure. Each sensor has one plate of the capacitor attached to a common substrate with the second plate of each capacitor being a metal diaphragm joined along its periphery to a common substrate. The two capacitors have an air dielectric therebetween, and when a change in pressure occurs across the metal diaphragm, the thickness of the air gap changes resulting in a change in capacitance for one of the capacitors.
Each of these systems has drawbacks, especially when a pressure change of a low order of magnitude is to be detected. The drawbacks include sensitivity to temperature changes, ambient temperature compensation, humidity sensitivity and a limited ability to withstand contaminants within the environment being measured Moreover, the prior devices are not suitable for pressure measurements in the low 0-1 inch of water range. These devices are also structurally complex and difficult to manufacture. Solutions to the problems associated with the prior art devices would represent significant advances in this technology.